1. Field of the Invention
This invention pertains to the field of nonwoven fabrics and the manufacture thereof. More specifically, it relates to such nonwoven fabrics that are comprised of one or more component layers of staple length or longer fibers or continuous filaments, for example, fabrics of microfibers having an average diameter in the range of up to about 10 microns. These microfiber webs have high capillarity and find many uses in absorbing oily materials such as, for example, industrial wipers, oil spill clean-up materials, and the like. When treated for wettability, these materials may also be used for absorbing aqueous liquids in applications such as institutional food service wipes and the like.
In such applications as well as in many others such as in garment and protective covers, these materials have found acceptance. However, for some purposes a high degree of conformability and softness is desired to avoid a papery feel and improve properties for wiping purposes, for example. The present invention relates to improvements in these and other properties and nonwoven materials incorporating such improvements as well as the methods for achieving them.
2. Description of the Prior Art
Thermoplastic nonwoven webs are well-known and have been produced by a number of processes including fibrillation, melt and solution spinning, and the like. One method that has achieved commercial acceptance is a meltblowing process developed by the Naval Research Laboratories which is described, for example, in Wendt, Industrial and Engineering Chemistry Volume 48, No. 8 (1965) pages 1342 through 1346. This process has been further developed by others and improvements are described in, for example, U.S. Pat. No. 3,978,185 to Buntin et al issued Aug. 31, 1976, U.S. Pat. No. 3,795,571 to Prentice issued Mar. 5, 1974, and U.S. Pat. No. 3,811,957 to Buntin issued May 21, 1974. The Buntin et al patent further discloses that mats of meltblown polyolefins are useful as wiping cloths and as hydrocarbon absorption material.
In general, such meltblown materials lack adequate strength for many applications. Accordingly, various steps have been investigated to increase strength properties of these webs. Including among these is the step of spot bonding by patterned adhesive application or by means of heat and pressure which is described, for example, in co-pending U.S. Pat. No. 4,307,143 to Meitner entitled "Microfiber Oil and Water Wipe," issued Dec. 22, l981. While improving strength properties, such spot bonding also increases stiffness and decreases bulk, both of which results are undesirable for many applications. Therefore, a need exists for thermoplastic fiber webs of improved softness, conformability and bulk while yet retaining at least to a high degree strength properties attributable to pattern bonding as well as other desired properties depending upon the intended application.
Stretching of nonwoven webs to improve strength properties is also known and described in the patent literature. For example, U.S. Pat. No. 3,772,417 to Vogt issued Nov. 13, 1973 describes such a process where thermally bonded nonwovens are stretched in the presence of heat in a two step process to give a total stretch that increases the area independent of the direction of stretch up to about 15 to 16 times or more. Drawing of meltblown microfiber webs is also described in U.S. Pat. No. 4,048,364 to Harding et al issued Sept. 13, 1977. As with the previously described Vogt patent, the purpose is to increase strength properties and the draw ratios are used from about 2 to 1 to about 10 to 1 under heated conditions, resulting in a ribbon useful, for example, in constructing a scrim support. U.S. Pat. No. 4,223,059 to Schwarz issued Sept. 16, 1980, discloses a process and apparatus for selectively stretching nonwovens. Other patents relating, in general, to stretching of nonwovens include U.S. Pat. No. 3,949,127 to Ostermeier et al issued Apr. 6, 1976 and U.S. Pat. No. 4,276,336 to Sabee issued June 30, 1981.